Touch-sensitive devices may detect touch via several different mechanisms, including but not limited to optical, resistive, and capacitive mechanisms. Some optical touch-sensitive devices detect touch by capturing an image of a backside of a touch screen via an image sensor, and then processing the image to detect objects located on the screen. Such devices may include an illuminant within the device to illuminate the backside of the display screen such that objects on the screen reflect the incident light toward the image sensor, thereby allowing the object to be detected.
One difficulty that may be encountered with optical touch screen devices involves differentiating between external (ambient) light and light reflected from the illuminant within the device. In general, the higher the ambient light level, the more difficult it may be to detect objects of interest. The ambient light forms a background noise floor that reduces contrast and makes it difficult to isolate the signal from the object. Correcting an image for ambient light may pose difficulties, as camera-based ambient light detection may be a fairly complex function depending on such factors as the ambient light source, size, distance from the camera, angle of incidence, spectral distribution, etc. Further, gauging the effect of ambient light on such a system may be difficult to do with a photometer or other such light meter in many cases.